There is an increasing demand to improve fuel economy, reduce emissions, and reduce noise levels of vehicles. As an alternative to the internal combustion engine (ICE), automotive manufacturers have developed hybrid powertrains that include both an electric traction motor and an internal combustion engine. During operation, HEVs use one or both of the power sources to improve efficiency.
The HEVs use either a parallel drivetrain configuration or a series drivetrain configuration. In the parallel HEV, the electric motor works in parallel with the ICE to combine the power and range advantages of the engine with the efficiency and the electrical regeneration capability of the electric motor. In the series HEV, the ICE drives an alternator to produce electricity for the electric motor, which drives a transaxle. This allows the electric motor to assume some of the power responsibilities of the ICE, thereby permitting the use of a smaller and more efficient engine.
In both configurations, the electric machine stores energy in batteries and uses the stored energy to power the vehicle. The HEV shuts down the ICE when the vehicle is stopped or idling. The electric machine propels the vehicle and eventually restarts the ICE. The electric machine stores braking energy in the batteries during regenerative braking.
The ICE in the HEV may include an engine control system that deactivates cylinders under low load situations. For example, an eight cylinder engine can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand (DOD). As used herein, an activated mode refers to operation using all of the engine cylinders. A deactivated mode refers to operation using less than all of the cylinders of the engine (one or more cylinders not active). An activation transition mode refers to a transition from the deactivated mode to the activated mode. A deactivation transition mode refers a to transition from the activated mode to the deactivated mode.
To smoothly transition between the activated and deactivated modes, the ICE preferably produces torque with a minimum of disturbances. Otherwise, the transitions will not be transparent to the driver. In other words, excess torque will cause engine surge and insufficient torque will cause engine sag, both of which degrade the driving experience.
Conventional spark retard techniques have been used to compensate for the momentary torque increase during the transitions. Retarding the spark delays the time to peak pressure which reduces the torque output. Such techniques are undesirable in that they reduce the overall torque output of the engine.